Thrust reversers for aircraft engines, particularly gas turbine engines, are well known and take many and varied forms.
One form of thrust reverser structure when used on a gas turbine engine having a by-pass fan and by-pass fan duct utilises so-called blocker doors which are activated by a cowl portion which translates rearwardly in a direction parallel to the engine axis and activates the blocker doors by means of an articulated linkage to substantially block the by-pass duct to propulsive fluid flow and redirect that fluid flow through a second outlet having flow directing cascades to provide reverse thrust to slow an aircraft when landing, for example. Whilst the by-pass fan duct may be contoured to optimise fluid flow therethrough when in normal propulsive mode to provide forward thrust, the blocker doors add weight as do the cascades which has implications for fuel economy. Furthermore, the articulated activating linkage also adds weight, is prone to wear and requires maintenance all of which add cost.
More recently, thrust reversers known as “natural blockage” thrust reversers have been successfully used. Examples of this type of thrust reverser are exemplified in GB-A-2 368 566 and EP-A-1 515 035 both of which are of common ownership herewith. In this type of thrust reverser, a translating cowl moves in a rearwardly direction parallel to the engine axis to open an outlet through which a fluid efflux emanates in a generally forwardly direction to provide reverse thrust. In this thrust reverser structure, the by-pass fan duct has a more exaggerated curved contour in the region of the reverse thrust fluid outlet when viewed in the radial direction so as to enable structure on the translating cowl itself to block fluid flow to the by-pass fan duct propulsive outlet. Thus, the cowl structure itself “naturally blocks” the by-pass fan duct and obviates the need for blocker doors and their associated structure and mechanisms. This results in a thrust reverser which is simpler and more reliable because complexity is removed with a consequent reduction in weight. Furthermore, this type of thrust reversing arrangement more efficiently blocks the by-pass fan duct than do the earlier blocker door arrangements. However, in order to make this type of thrust reverser operate, the by-pass fan duct must have a more pronounced curvature or profile resulting in the surface area of the by-pass fan duct being increased which causes a small weight increase but, more importantly, causes pressure losses in the fan duct that may reduce forward thrust performance and consequently increase fuel burn. These considerations become increasingly important as engine size increases. However, this type of thrust reverser has been successfully used on commercial regional jet aircraft.